1. Field of Invention
This invention relates to thermally-responsive record material. It more particularly relates to such record material in the form of sheets coated with color-forming systems comprising chromogenic material (electron-donating dye precursors) and acidic color developer material. This invention particularly concerns a thermally-responsive record material capable of forming a non-reversible image resistant to fade or erasure due to contact with oils, solvents or exposure to elevated temperature. The invention teaches a record material having improved image density retention.
2. Description of Related Art
Thermally-responsive record material systems are well known in the art and are described in many patents, for example, U.S. Pat. Nos. 539,375; 3,674,535; 3,746,675; 4,151,748; 4,181,771; 4,246,318; and 4,470,057 which are incorporated herein by reference. In these systems, basic chromogenic material and acidic color developer material are contained in a coating on a substrate which, when heated to a suitable temperature, melts or softens to permit said materials to react, thereby producing a colored mark.
Thermally-responsive record materials have characteristic thermal responses, desirably producing a colored image of sufficient intensity upon selective thermal exposure. Improvements in thermal response would be of commercial significance.
A drawback of thermally-responsive record materials limiting utilization in certain environments and applications has been the undesirable tendency of thermally-responsive record materials upon forming an image to not retain that image in its original integrity over time when the thermally-responsive record material is exposed to elevated temperatures in use or storage. As a result, due care and control in handling imaged thermally-responsive record materials has been required. This loss of image density or fade can be annoying and detrimental whenever the integrity of records is allowed to become suspect through improper record storage.
The ability of a thermally-responsive record material to resist image decline upon exposure to elevated temperatures would be an advance in the art and of commercial significance.
It is an object of the present invention to disclose a thermally-responsive record material having improved image retention at elevated temperatures, and/or improved thermal response.